1. Field of the Invention
This invention relates to drive systems for controlling transport belts in production freezing systems, and more particularly to such drive systems in spiral freezer apparatus in which the conveying mechanism is commonly known as spiral.
2. Related Art
Lotension spiral drive systems rely on overdrive to operate properly, wherein the relationship between the speed of the belt at the inside edge and the surface of the drum is very important in the spiral operation.
The overdrive condition is usually accomplished by a mechanical linkage between the cage and the take-up. The linkage is limited in adjustment by gears and sprockets and the resulting ratios available. Such an arrangement is fixed until the ratios are physically changed. The mechanical linkage is usually cumbersome and difficult to implement. The take-up is, by association, too strong and can cause damage to the belt in a jam (clutch systems are unreliable to solve such a problem). The mechanical drive on the drum (chain oriented horizontally) and the mechanical linkage require relatively high maintenance for the chain, sprockets, tensioners, bearings and chain guides.
U.S. Pat. No. 3,280,393 discloses a tension override used in a tensioning motor control system that operates in addition to the normal tension controls associated with paper processing machines and which senses changes in the web tension. One of the supply roll or the calendar roll is adapted to move in response to changes in tension in the web. An electrical signal representing the change in web tension is input to a regulator controlling the field of the unwind generator to either increase or decrease the braking force applied to the unwind motor of the supply roll.
U.S. Pat. No. 4,126,817 is another tension control servo system for maintaining constant tension on a moving web used to transport and position original documents in an exposure station of a reproduction machine. A take-up reel motor and supply reel motor are energized by a signal from a common source, with the signal applied to the supply reel motor being a derivative of that applied to the take-up reel motor. The supply reel is thereby energized during periods of acceleration or deceleration of the web.
The web tensioning system of U.S. Pat. No. 4,363,457 also maintains constant tension of a web between a supply and take-up reel. The currents are sensed in each of the motors driving the supply reel and the take-up reel such that the sum of the motor currents is maintained at a substantially constant value, whereby the sum of the torques generated by the motors is held substantially constant with changing radii of the spools during web movement.
The web movement control of U.S. Pat. No. 3,809,335 uses a plurality of error signals, including a tension command pulse, which are individually weighted for each of the two reel motors such that the reel motor servocontrol mechanism controls the motors to obtain desired tape acceleration/deceleration, speed and tension parameters.
The reel servocontrol system of U.S. Pat. No. 4,513,229 uses a motor-driven capstan of low inertia to draw tape past a cylindrical scanning drum and a tension servo arm to provide a signal representative of tape tension in a loop adjacent the supply reel. A tension error signal controls the supply reel motor and a tension reference signal is modified in response to a signal which represents the energization of the capstan motor, and accordingly the torque output thereof to provide automatic compensation for variation in tape tension at the scanning drum.